The present invention relates to a system and method for initial synchronization of a steering wheel and road wheels in a steer-by-wire system for vehicles.
In vehicle steer-by-wire systems, mechanical linkages between the steering wheel and the front road wheels typically are eliminated. Moreover, the mechanical linkages between the two front road wheels are eliminated in some steer-by-wire systems. Rather, two independent road wheel electric motor actuators may be installed on the vehicle wherein each actuator independently actuates one of the front road wheels. This allows the two front road wheels to be moved independently from each other. Moreover, a steering wheel system based on an electric motor actuator may also be used for providing steering feel and steering command to the road wheels.
Initial synchronization of the steering wheel and road wheels is a basic functional requirement of a vehicle steer-by-wire system. In a typical steer-by-wire system, the initial angular position of the steering wheel and the road wheels are uncertain when the system is first powered. If the steering wheel and road wheels are not initially aligned in a steer-by-wire system, the steering performance of the vehicle will be degraded and the vehicle may not even be operable. Therefore, an initial synchronization or an alignment of the steering wheel and road wheels is necessary to implement in a steer-by-wire system control. Generally, the initial synchronization of the steer-by-wire system in this invention is a process to align the steering wheel and road wheels for operation of the vehicle when a vehicle is first powered.
An ideal initial synchronization process must execute rapidly, must not cause discomfort to the driver and passengers, and must be interruptible by the vehicle driver without any adverse effect to the initial synchronization process. Since these requirements are in conflict, a number of issues may arise during the startup of the vehicle.
Typically, steer-by-wire systems need a certain amount of time to synchronize the steering wheel and the road wheels when it is initially powered. In particular, the initial synchronization time is longer if the initial error between the two road wheels or between the steering wheel and the road wheels is large. Thus, a driver may be required to wait for the initial synchronization process to be completed before the vehicle may be driven away.
To reduce the synchronization process time, the steering wheel and road wheels should be turned to the desired angular positions very rapidly. Thus, the comfort issue for the driver and passengers arises during initial synchronization. Rapid response time means rapid movement of the steering wheel and road wheels. Rapid movement of the road wheels causes a sudden jolt, which may be uncomfortable to the occupants of the vehicle.
Additionally, initial synchronization process may be interrupted by the vehicle driver. The driver may hold and turn the steering wheel when the vehicle is powered. This causes a disturbance and interruption to the initial synchronization process. It may cause the steering wheel and/or road wheels to not align. Therefore, the initial synchronization process is required to operate continuously in the presence of driver interruption.
It is a challenge to realize an initial synchronization strategy with rapid response time, comfortable feel, and driver interruptible function. These requirements are in conflict. It is difficult to satisfy these requirements by only using simple trade-offs among the requirements in the initial synchronization process of steer-by-wire systems.